1. Field of the Invention
The present invention relates to an optically amplified gain control circuit and an optically amplified gain control method, and in particular, to those for controlling the gain of an optical amplifier.
2. Description of the Related Art
Conventionally, an optically amplified gain control circuit and an optically amplified gain control method are applied for an optical amplification of an optical communication system. Nowadays, as the Internet is becoming common at an explosive pace, it is necessary to drastically increase the transmission capacity of a backbone transmission system. As a means for increasing the transmission capacity effectively using the existing infrastructure, DWDM (Dense Wavelength Division Multiplexing) is becoming attractive. As a means for totally amplifying an optical signal that has been wavelength multiplexed, an optical amplifier is becoming important.
FIG. 1 shows an example of the structure of a conventional optical amplifier. In FIG. 1, an optical signal (composed of a main signal 101 and an SV signal 102) is input from a transmission path. The input optical signal is partly branched by a directional coupler 70. The branched optical signal is converted into an electric signal corresponding to the power of the input optical signal by a photoelectrically converting device 75. In addition, a part of an output of an optically amplifying medium 20 is branched by a directional coupler 74. A photoelectrically converting device 78 converts the output of the directional coupler 74 into an electric signal corresponding to the power of an output signal to the transmission path.
A control circuit 60 controls power of exciting light of a forward exciting light source 76 and a backward exciting light source 77 so as to control the output power to the transmission path to a predetermined value.
However, since the power of the optical signal on the transmission path varies corresponding to the number of signals that have been wavelength multiplexed, the output power to the transmission path should be varied corresponding to the number of multiplexed signals.
To do that, conventionally, a control circuit 60 superimposes a signal that represents the number of multiplexed wavelengths with the SV signal 102 so that a predetermined optical output power is transmitted to the transmission path corresponding to the number of multiplexed wavelengths.
However, in the related art reference, since the amplified gain of an optical amplifier is not flat in the amplification range, the optical power may not be optimally controlled depending on the position of the wavelength used as the main signal.
An object of the present invention is to provide an optically amplified gain control circuit and an optically amplified gain control method that allow the flatness of an amplification of a multiplexed signal to improve.
To accomplish the above-described object, the present invention is directed to an optically amplified gain control circuit, comprising an optically amplifying medium for amplifying an optical signal, a exciting light coupler for combining exciting light with the optical signal optically amplified by the optically amplifying medium, and a control circuit for controlling an output of the exciting light combined with the optical signal by the exciting light coupler, wherein the optical signal to be amplified is composed of a main signal, an SV signal, and a tone signal, which are multiplexed, and wherein the control circuit controls the output of the exciting light corresponding to the tone signal on the preceding stage of the optically amplifying medium and the tone signal on the next stage of the optically amplifying medium so as to control the amplification of the optical signal by the optically amplifying medium.
The output of the exciting light combined with the optical signal is controlled corresponding to the wavelength of the tone signal. The optically amplified gain control circuit further comprises two directional couplers for branching the optical signal so as to obtain the tone signal for controlling the output of the exciting light, wherein the directional couplers are disposed on the preceding stage of the optically amplifying medium and on the next stage of the optically amplifying medium. The optically amplified gain control circuit further comprises two band pass filters disposed on the preceding stage of the optically amplifying medium and on the next stage of the optically amplifying medium, for obtaining the tone signal from the optical signal branched by the directional couplers disposed on the preceding stage of the optically amplifying medium and the next stage of the optically amplifying medium. The wavelength of the tone signal is at nearly the center of the wavelength range of the optical signal.